Dust collection facilities operating at cement or lime producing kilns remove particulate matter from the kiln exhaust. This particulate matter includes calcium carbonate, calcium oxide, and the oxides and carbonates of other metals, depending on the composition of the feed stock to the kiln. Two elements frequently present in the kiln dust are potassium and sodium. These elements limit or prevent the reuse of the kiln dust as kiln feed stock, since they interfere with the properties of the final product, and the dust is therefore discarded. These dust collection facilities do not remove sufficient gaseous pollutants from the exhaust stream and separate scrubbers must be provided if they are to be prevented from entering the atmosphere.
The use of alkali or alkaline earth materials, as solids or in a slurry or in a solution, for scrubbing (removing sulfur and nitrogen oxides) of exhaust gas has been known for years (see discussion in Morrison, G., supra, hereby incorporated by reference). For example, Mehlmann (1985, Zement-Kalk-Gips Edition B) describes the use of hydrated or pulverized limestone at temperatures up to 1100.degree. C., or of spray drying with hydrated lime; and Ayer (1979, EPA-600/7-79-167b) describes the use of lime to scrub exhaust gas from a heating plant. Limestone may be included in the charge of fluidized bed furnaces for the same purpose. In general the oxides of carbon, sulfur, and nitrogen, present in exhaust gas, when reacted with water, produce acids, including sulfuric, sulfurous, nitric, and carbonic. The presence and amounts of each depend on the oxides present, the availability of oxygen, and the reaction conditions. When these acids are reacted with the oxides, hydroxides, or carbonates of alkali or alkaline earth metals, salts of the components are formed. For example, sulfuric acid will react with the calcium carbonate present in limestone to yield calcium sulfate.
The principal object of the present invention is to provide a system for eliminating the dust disposal problem present in cement plants and simultaneously reducing gaseous and particulate emissions and to be able to do so cost effectively, including the conversion of otherwise waste products into useful products.